1. Field of the Invention
The present invention relates to an active material, a nonaqueous electrolyte battery, a battery pack and a vehicle.
2. Description of the Related Art
As one of the trends in improvements in the performance of nonaqueous electrolyte batteries such as lithium secondary batteries, there is an improvement in reliability in, for example, long-term storage performance. To improve the reliability of such batteries, it is crucial to reduce the deterioration caused by a side reaction between a nonaqueous electrolyte and an electrode active material. As a method of significantly suppressing the side reaction, there is a method in which the charge potential of a negative electrode is raised. The potential of the negative electrode during charging is raised not by using an active material like graphite which charges and discharges at a potential close to the ionization potential of a lithium metal but by using, as a negative electrode, an active material which undergoes a lithium-absorption/release reaction at a higher potential. The side reaction in the nonaqueous electrolyte scarcely progresses by the use of such an active material. Negative electrode active materials used to achieve such ends are desired to enable the release and absorption of lithium at a potential range of 0.5 to 2 V (vs. Li/Li+).
Although oxides such as Li4Ti5O12 and TiO2 enable the release and absorption of lithium at a potential range of 0.4 to 2.5 V (vs. Li/Li+), they provide unsatisfactory capacity and cycle performance. Lithium titanate such as Li4Ti5O12 has high reliability in the lithium-absorption/release reaction. However, the capacity per weight of lithium titanate is about ½ that of graphite at the most. Therefore, the energy density of a lithium secondary battery using lithium titanate is lower than that of a lithium secondary battery using the graphite. It is therefore necessary to use a new active material having a large capacity to make progress in higher energy densification of a battery. However, there is the problem that, for example, TiO2 having a higher capacity density per unit weight than lithium titanate is increased in cycle deterioration. Three documents explained below disclose the technologies concerning these active materials, to limit cycle deterioration caused by addition of other metal elements. However, all of these batteries are deteriorated not only in cycle performance but also in capacity.
In JP-A 2004-235144 (KOKAI), a lithium-transition metal composite oxide having a spinel structure containing an alkali metal and/or alkali earth metal and specifically, a lithium-transition metal composite oxide represented by the formula: LiaTibMdO4+c (M represents at least one element selected from the group consisting of a II-group, XIII-group or XIV-group metal in the periodic chart, a halogen atom, sulfur and transition metals except for titanium, 0.8≦a+d≦1.5, 1.5≦b≦2.2, 0≦d≦0.1 and −0.5≦c≦0.5) is used as the negative electrode active material for a nonaqueous electrolyte battery.
JP-A 2000-268822 (KOKAI) discloses that a composite oxide is used as the active material of the positive electrode or negative electrode. The composite oxide contains a phase of an anatase type crystal structure, and is represented by the formula: MXTi1-XO2 (M represents at least one of V, Mn, Fe, Co, Ni, Mo and Ir, and 0≦X≦0.11).
In the meantime, JP-A 7-230800 (KOKAI) discloses that a composite oxide represented by the formula: LixSi1-yMyOz (0≦x, 0<y<1, 0<z<2, M represents a metal excluding an alkali metal or a similar metal excluding silicon) is used as the negative electrode active material of a nonaqueous electrolyte secondary battery. JP-A 7-230800 (KOKAI) discloses a method in which a composite oxide is produced by treating a starting material to heating in an atmosphere excluding oxygen, such as an inert gas atmosphere or under vacuum. This method enables controlling the quantity of oxygen or the partial pressure of oxygen in the heat treatment atmosphere, thereby making it easy to yield the composite oxide.